Two incubation temperatures ...

Two incubation temperatures and times are used: 37 °C for 24 hours to encourage the growth of bacteria of mammalian origin, and 22 °C for 72 hours to enumerate bacteria that are derived principally from environmental sources. 

The colony count at 22 degrees C generally represents those bacteria naturally present in water and are not of sanitary significance. 

Maximum Acceptable Concentration for  Drinking Water  = none detectable per 100 mL This means that in order to conform to the guideline: For every 100 mL of  drinking water  tested, no total  coliforms  or E.coli should be detected.